1. Field of the Invention
The present invention relates to an infrared radiation sensitive device, and more particularly to an infrared radiation sensitive device comprising a pyroelectric unit, a light receiving face on said pyroelectric unit for receiving infrared light radiated from an object to be detected, and a pair of external electrodes on opposite end surfaces of said pyroelectric unit respectively.
2. Description of the Prior Art
One prior known pyroelectric infrared sensor device 1A is shown in FIG. 7A. This prior art device includes a rectangular-plate-shaped pyroelectric body 1a having top and bottom surfaces on which electrodes 2a, 3a are formed respectively with polarization treatment being applied between the electrodes 2a, 3a in the direction designated by the arrows shown in FIG. 7A. Another prior art pyroelectric infrared sensor device 1B is shown in FIG. 7B which includes a pyroelectric body 1b having side surfaces on both of which electrodes 2b, 3b are formed with polarization treatment being carried out between the electrodes 2b, 3b as indicated by the arrows in FIG. 7B. With these pyroelectric infrared sensor devices 1, the electrodes 2a, 3a and 2b, 3b are each formed in the direction perpendicular to the polarization direction as designated by the arrows.
In these prior art pyroelectric infrared sensor devices 1A and 1B, when infrared light rays 4 radiated from an object to be detected are incident on the light receiving plane of each pyroelectric infrared sensor device, a slight temperature change takes place in the respective pyroelectric body 1a, 1b. Such temperature change results in a packet of surface charge carriers in the pyroelectric body 1a, 1b, which carriers have been in the equilibrium condition, being forced to move, causing a corresponding voltage to be generated. This voltage is used to detect or sense the object being detected, by electrically amplifying the voltage with an associated impedance conversion circuit employing field effect transistors (FETs) and thereby converting the voltage into a corresponding electrical signal.
In order for such prior art pyroelectric infrared sensor devices to have improved relative detection rate or "detectivity" indicative of the pyroelectric device characteristics, it has been required that thermal insulation be attained by decreasing the thickness of the pyroelectric body per se while simultaneously causing the light receiving sections which are expected to receive incoming infrared light rays to be formed in a hollow shape. This allows the thermal or heat capacitance to decrease with the thermal time constant being decreased as low as possible thus improving the resultant relative detectivity.
Unfortunately, the aforesaid prior art pyroelectric infrared sensor devices encounter several difficulties, which follow. Since it has been strictly required to obtain thermal insulation while decreasing the thickness of the pyroelectric body, the mechanical strength of such pyroelectric body itself might be weakened so as to reduce its durability against application of thermal shocks, thereby making the handling of such pyroelectric infrared sensor devices difficult. In other words, if the pyroelectric body is decreased in thickness, then certain risks might arise: mechanical damage including micro-cracks can occur in the pyroelectric infrared sensor devices per se due to significant thermal variations upon application of external vibrations and shocks as well as upon irradiation of strong infrared light rays thereon, while the performance thereof can decrease due to degradation of heat flow balance. Furthermore, since the pyroelectric body per se becomes brittle, making it difficult for the pyroelectric body to be reliably supported by adhesion on a substrate, which in turn makes it very difficult both to settle its support method and to attain accurate position alignment accuracy. Due to the above difficulties the resulting performance can decrease undesirably.
Another problem faced by the prior art is that since in most cases the thickness of the pyroelectric body itself is designed to fall within the range of from 70 to 100 micrometers, the production yield remains low, rendering it extremely difficult to manufacture or fabricate the pyroelectric infrared sensor devices per se. This results in the manufacturing costs of the prior art pyroelectric infrared sensor devices being undesirably increased.
In a prior art approach to avoidance of the aforementioned problems, it is attempted to specifically arrange electrodes of a pyroelectric infrared sensor device so that polarization of a pyroelectric body thereof occurs only near a light incidence plane (a light receiving plane) of the device. One exemplary pyroelectric infrared sensor device 5 employing such a scheme is shown in FIG. 8. This structure comes in view of the fact that the contribution of the top surface of the light receiving plane occupies most part of an output of pyroelectric infrared sensor devices.
More specifically, the pyroelectric infrared sensor device 5 shown in FIG. 8 includes, for example, a rectangular plate-shaped pyroelectric body 6 having a top surface on which two opposed electrodes 7, 8 are formed with a certain distance D being defined therebetween. Polarization treatment has been effected by applying a direct current (DC) voltage between the two electrodes 7, 8. In this case, polarization is generated mainly at a section positionally close to the light incidence plane of the infrared light rays 9 with the polarization direction being parallel to the light incidence plane of the infrared light rays 9; in other words, the polarization direction is identical to the x-axis direction of FIG. 8.
Accordingly, this pyroelectric infrared sensor device 5 is superior to those shown in FIGS. 7A and 7B in that the polarization of its pyroelectric body 6 is restricted to being at or near the light incidence plane (light receiving plane) of the pyroelectric infrared sensor device 6, thereby improving the relative detectivity.
However, the above advantage of the prior art pyroelectric infrared sensor device 5 does not come without an accompanying penalty: a reduction in controllability of the relative detectivity. More specifically, in the prior art pyroelectric infrared sensor device 5, it should be required in order to change the relative detectivity, the light receiving area of the pyroelectric body 6 for receiving infrared light rays must be changed, which results in increasing complexity and time. For this reason, the prior art device is not adaptable for use in stable mass production while at the same time it is rendered difficult to manufacture the same at low cost.